Electric cable



Jan. 12, 1937- c. J. BEAVER ET AL.

ELECTRIC CABLE Filed Aug. l5, 1953 Figl.

Figz. 9

1MMATTO/Muna Patented Jan. 12, 1937- UNITEDI STATES ELECTRIC CABLECharles James Beaver and Edward Leslie Davey,

Cheshire, England, assignors to W. T. Glover and Company Limited,Manchester, England, a

British company Application August 15,

In Great Britain 11 Claims.

It is well known that in high voltage cables of the paper insulated leadcovered type, as at present manufactured, the presence of remanent oroccluded air or gas is very undesirable because of the risk of it beingionized at high electric stresses and thereby producing local conditionstending to the breakdown of the dielectric.

By the common method of impregnation, in which the cable', previouslylapped with paper, is dried and impregnated in vacuo with oil orcompound (hereinafter, for convenience, spoken of as compound), it isrecognized that it is impossible to eliminate entirely residual air andgas.

Further, this method imposes comparatively low limits on the viscosityof the impregnating compound in order that it may penetrate the wall ofdielectric. I

The result is that, while the residual air or gas 20 is usually finelydivided and uniformly distributed as very small bubbles or in solutionin the impregnating compound of the dielectric when the cable is newlymade, and the dimensions of the occlusions are such that the gas is notionizable until much higher stresses than those corresponding to theworking voltage have been applied, the said dimensions are liable tobecome altered by aggregation after the cable has been heated, so thationization may then occur at 30 lower stresses.

This aggregation is permitted by the mobile character ofthe-impregnating compound, which is necessitated, Vas mentioned, by therequirements oi.' penetration in the' impregnation process.

0n the other hand if itis attempted to impregnate more fully, sothat:residual gas is much reduced, the dilation of the impregnatingcompound-unrelieved by gaseous compression in the dielectric-distendsthe lead sheath when the cable is heated under load, so that on cooling,low pressure spaces, containing gas ionizable under working conditions,tend to form in the dielectric. Y

In order to overcome'these drawbacks of lead distension and/0r gasaggregation, producing conditions under which the residual gas in animpregnated paper dielectric may become ionizable under workingconditions, in accordance with the present invention anew combination ofconditions within the cable is provided. One of the conditions is thatspaces of definite and substantially invariable dimensions are providedfor the gas present in the cable. The other condition is that the gas iscontinuously maintained under a pressure substantially below atmosphericpres- 1933, Serial No. 685,192 September 27, 1932 sure. It is well knownthat for each thickness of gas layer the ionization voltage fallscontinuously as the pressure of the gas is reduced until a criticalpressure (hereinafter called the criticalv value) is reached; below thispressure the ionization voltage rises rapidly. The pressure in thepresent case is made so low, having regard to the dimensions of thespaces, that the gas therein will not. be ionizable in the conditionsunder which the cable will work. This pressure is ad- 10 justed to bebelow the critical value for the largest radial dimension of any of thegas spaces in the cable so as to take advantage of the relatively highionizationV voltage obtained in that region.

'I'he invention is applicable both to cases where the paper isimpregnated and to cases where the paper is unimpregnated. It is alsoapplicable to other dielectrics than paper which are applied in asimilar manner, for instance, to the material 20 known as varnishedcambric. For convenience in the following description the soliddielectric material will be spoken of as paper.l

The dimensions of the spaces are assured by forming the spaces in theprocess of lappingv on the paper layers so that they are determinedradially by the thickness of the paper, and longitudinally by thespacing of the lappings in each layer. Where impregnated paper is vto beused steps are taken to provide that the spaces between the lappings ofpaper are substantially clear of free compound, that is the compound isvretained. in the paper and on its surface.

In preparing impregnated paper for use in the manufacture of a cable inaccordance with the present invention it is impregnated beforeapplication to the conductor with a compound hav-l ing such a settingpoint as to ensure that physical stability of the impregnated paper ismaintained up to a temperature well above the maximum 40 reached in theworking conditions of the cable. The impregnation may, for instance, becarried out as set out in the speciiication of U. S. Patent 1,752,972 orof U. S. Patent No. 1,958,984.

In addition to the impregnation the paper may be provided `^with alubricating film as described in the said specifications. This film ispreferably applied to one side of the paper Aonly and is of very smallthickness. It may, for instance, consist of a coating of petroleum jellyhaving a. high viscosity and set point.

The impregnating treatment of the cable is completed by the operationson the paper above described. There is no subsequent addition of encenumerals 3, I and 5.

compound, either during the lapping of the paper or to the completedpaper dielectric.

An example oi a cable embodying the present invention is illustrateddiagrammatically in the accompanying drawing of which Figure l shows atransverse section and Figure 2 shows a longitudinal section.

In this cable the conductor I is covered with a thin metal tape or foil2 and is lapped with layers or lamlnae of paper strip indicated by therefer- Over the outside of this dielectric material is placed ametallized layer 6 (preferably perforated) and over this the lead sheathl is applied. To the outside of the lead sheath is applied a metalreinforcing tape 8 and this is protected in the usual way by layers ofimpregnated fabric 9.

The metal tape 2, the metallized paper 6, the reinforcing tape 8 and thefabric layers 9 will as usual be lapped on helically. No attempt hasbeen made to show this in the drawing.

The thin metal tape 2, lapped on the conductor I, performs the knownfunction of reducing local intensities of electric stress and also givesa definite inner cylindrical surface to the di electric layer.

Instead of using this tape the desired results can be obtained byforming (either before or after stranding) the component wires of theoutermost layer of the conductor in such a way as to give a smooth outersurface to the conductor. Alternatively a thin lead tube covering theconductor as a whole may be used as described in the specification of U.S. Patent No. 1,256,863.

It will be seen from the drawing how the dimensions of the gas spacesIIJ, between adjacent turns of the paper are fixed by (l) the spacingapart of adjacent turns and 4(2) by the thickness ofthe paper laminae.The former determines the longitudinal width of the spaces and thelatter, the radial depth thereof. The metal tape 2 forms the innersurface of the innermost gas spaces. The metallized paper layer 6 formsthe outer surface of the outermost gas spaces It will be understood thatany gas which may be found in spaces between the metallized paper layer6 and the sheath 1 will not be subject to electric stress. The sameremark applies to the gas between the tape 2 and the wires of theconductor i and in the interior of this conductor.

From the preceding description of a single core cable it will be readilyrecognized how the invention may be embodied in cables having more thanone core. It will ybe understood that the precautions taken to securethe absence of free compound must be extended to the material used forthe nllers between the cores (where these are irnpregnated) as well asto any belt insulation which may be used in addition to the insulationof the individual cores.

Accurate predetermination of the spaces for the gas formed betweenadjacent turns of each helically applied layer of impregnated orunimpregnated paper results from the choice of a suitable thickness ofpaper and the staggering of successive layers so that none of thehelical spaces are directly superposed. This last condition can beattained, for instance, by lapping the layers of impregnated paper onthe cable by means of a machine of the type in which the cable rotatesand the heads from which the paper is :laid out `re stationary and therelative positi ns `1f Th p nar stripz.: accurately maintainable :nuncle n inuous observation Such a machineildescribedinl A PatentNo1,591736 aoemvo It is to be understood however that the immersion of thecable in compound during lapping, which is described in thatspecification, is not to be employed in the present case. The strips ofthe paper are lapped on the cable in air which isI 5 preferably keptbelow normal humidity and the cable is then sheathed with lead or leadalloy, to which reinforcement may be applied as protection againstexternal influences. After this the air enclosed in the dielectric,which is chiefly located l0 in the spaces provided between the turns ofpaper, together with the other air in the cable, is evacuated to thedesired extent. It may be replaced by an inert gas, such as nitrogen,carbon dioxide, helium, neon or other suitable gas. The l5 process ofremoval of air and filling with gas may be facilitated by a repeatedflooding and exhausting of the gas into and from the cable and by theapplication of heat to the cable intermittently or continuously, eitherby external means or by the passage of current through the conductor orconductors or by both of these. The final step in all cases is toevacuate the cable to obtain the dedesired low pressure, the cablehaving been sealed at the ends in order to prevent leakage inward.

The choice of the radial dimensions of the gas spaces and of the gaspressure will depend upon a number of conditions. It will howevergenerally be advantageous to make the gas spaces at the surface of theconductor of small radial 3U dimensions, for instance, one and a halfthousandths of an inch. By increasing the thickness of the layers orwrappings of paper from the conductor outwards, the radial dimensions ofthe gas spaces may also be increased, taking advantage 35 of the factthat the intensity of electric stress decreases from the surface of theconductor outwards. With such dimensions and with an absolute pressureof the order of 0-10 millimetres of mercury, conditions can be obtainedwhich per mit a very high voltage to be applied without obtainingionization. It will be understood that the figures given are only by wayof example.

Under no working conditions can the internal pressure exceed theexternal atmospheric pressure. The expansion of the solid dielectricwhich may occur will be compensated by a comparatively small change ofpressure in the gas spaces. Accordingly no sheath distention can occurand no reinforcement oftthe sheath to deal with this will be required.

Whatwe claim as our invention and desire to secure by Letters Patentiszi. An electric power cable comprising a conductor, an enclosingsheath and a laminated dielectric layer between the conductor and thesheath, said dielectric having spaces of predetermined dimensions andpositions distributed through the laminations, these spaces having aradial depth which increases from the conductor outwards, and gas inthese spaces at a pressure less than thecritical value for the largestradial dimension of any of said spaces.

2. An electric power cable comprising a conductor, an enclosing sheathand a laminated dielectric layer between the conductor and the sheath,said dielectric having spaced apart interstices in the laminations atpredetermined intervals and of predetermined sizes, the radial depth ofsaid interstices increasing from the conductor outwards, and gas in saidinterstices at a pressure below atmospheric and less than the critical.value for the largest radial dimension of any of said interstices.

3. Anelectriccablecomprilingaeonductonan n enclosing sheath and adielectric material between the conductor and the sheath, said materialbeing constituted by turns and laminae of paper tape with spaces ofdefinite and substantially invariable dimensions between the turns andof`increasing radial dimensions from the conductor outwards, said tapebeing fully impregnated with highly viscous compound which is conned tothe paper under all working conditions, and gas occupying all saidspaces, said gas being at a pressure substantially below atmospheric iand less than the critical value of the largest radial dimension for anyof said spaces.

4. An electric cable comprising a conductor, an enclosing sheath and adielectric material between the conductor and the sheath, said materialbeing constituted by turns and laminae of paper tape with spaces ofdefinite and substantially invariable dimensions between the turns andof increasing radial dimensions from the conductor outwards, said tapebeing fullyimpregnated with oil compound which has a setting point abovethe working temperature of the cable, and gas occupying all the saidspaces, said gas being at a pressure substantially below atmospheric andless than the critical value for the largest radial dimensions of saidspaces.

5. An electric cable comprising a conductor, an enclosing sheath and adielectric layer between the conductor and the sheath, said layercomprising wrapped paper impregnated with highly viscous compound andgas at a pressure substantially below that of the atmosphere, which gasoccupies spaces in the dielectric layer of dimensions predetermined insize so as to avoid ionization of gas and which spaces are substantiallyinvariable in position and dimensions and are substantially clear offree compound under all working conditions, which spaces aredistribuated through the dielectric layers.

6. An electric cable comprising a conductor, an enclosing sheath and alaminated dielectric layer between the conductor and the sheath, saidlayer comprising wrapped paper impregnated with highly viscous compound,and'gas, which gas occupies spaces ln the dielectric layer ofsubstantially invariable dimensions which are bounded by the turns andlaminae of the paper, which spaces are substantially clear of freecompound under all working conditions, said gas being at a pressuresubstantially below that of the atmosphere and less than the criticalvalue for the largest radial dimension of any of said spaces.

'7. An electric cable comprising a conductor, and enclosing sheath and alaminated dielectric layer between the conductor and the sheath, saidlayer comprising paper impregnated with a compound which lhas a highsetting temperature above the range of the working temperature of thecable and which compound is thereby retained in and confined to thepaper under all working conditions, said layer also comprising gas,which gas occupies spaces of substantially invariable dimensions, whichspaces are bounded by the turns and laminae of the paper, said gas beingat a pressure substantially below that of the atmosphere and less thanthe critical value for the largest radial dimension of any of saidspaces.

8. An electric cable comprising a conductor,

an enclosing sheath and a dielectric layer b'etween the conductor andthe sheath, said dielectric layer comprising wrapped paper impregnatedwith a highly viscous compound, said layer having helical spaces thereinof mechanically predetermined locations and dimensions such that thedielectric strength ofv any radial dimension of said layer ispredetermined,said spaces having gas therein at a pressure substantiallybelow atmospheric and less than the critical value for the largestradial dimension of any of said spaces, the spaces being formed betweenedges of adjacent turns of paper and between surfaces of concentricwrappings.

9. An electric cable comprising a conductor, an enclosing sheath and adielectric layer between the conductor and the sheath, said layer havinghelical spaces therein of mechanically predetermined locations anddimensions, said dielectric layer comprising wrapped paper impregnatedwith a highly viscous compound, the spaces being formed between edges ofadjacent turns of paper and between surfaces of concentric wrappings,said spaces having gas therein at a pressure substantially belowatmospheric and below the critical value for the largest radialdimension of said gas spaces, the dielectric strength of each part ofsaid layer being predetermined by the dimensions of the spaces and thegas pressure.

10. An electric cable comprising a conductor, an enclosing sheath anddielectric material between the conductor and the sheath, said materialbeing constituted by definitely spaced turns and layers of paper tape ofpredetermined thickness, which tape is fully impregnated with a highlyviscous compound which is confined to the paper under all workingconditions, all the spaces between the turns and layers of paper tapebeing occupied by gas at a pressure substantially below that of theatmosphere and below the critical value for the largest radial dimensionof any of said spaces.

11. An electric cable comprising a conductor, an enclosing sheath anddielectric material between the conductor and the sheath, said materialbeing constituted by denitely spaced turns and layers of paper tape ofpredetermined thickness, which tape is fully impregnated with oilcompound which has a setting point higher than the working temperatureof the cable, whereby the compound is conned to the paper under allworking conditions, all the spaces between the turns and layers of papertape being lled with gas at a pressure substantially below that of theatmosphere and below the critical value for the largest radialdimensions of any of the spaces.

CHARLES JAMES BEAVER. EDWARD LESLIE DAVEY.

